1. Field of the Invention
This invention relates to a GaN single crystal substrate for making blue light LEDs (light emitting diodes) or blue light LDs (laser diodes) based upon III-V group nitride (GaN-like) semiconductors.
This application claims the priority of Japanese Patent Applications No.10-171276(171276/1998) filed on Jun. 18, 1998 and No.10-183446(183446/1998) filed on Jun. 30, 1998, which are incorporated herein by reference.
2. Description of Prior Art
FIG. 1 shows the ratios of the lattice constant and the thermal expansion coefficient of the candidate materials for the substrate of growing GaN to those of GaN. Here, candidate substrates for GaN growth are sapphire (Al2O3), silicon carbide (SiC), silicon (Si), gallium arsenide (GaAs) and zinc oxide (ZnO). The nitride semiconductor light source devices, or GaN-group light source devices have been produced by piling GaN or GaN-like films epitaxially upon a sapphire substrate (Al2O3). The sapphire substrate is endowed with high chemical stability and high heat resistance. Sapphire is rigid. Although the lattice constant of sapphire is different from that of GaN by about 16%, GaN crystal can grow epitaxially on a sapphire substrate by interposing a GaN buffer film. For this reason, sapphire is exclusively used as a substrate for the GaN crystal growth. Since there is no means of eliminating the sapphire substrate, the GaN-group LEDs having the sapphire substrate are utilized for practical purposes. The current GaN-LEDs (more strictly; GaInN-LEDs) are complex devices including GaN and sapphire. The GaN/sapphire LEDs are already practical devices on the market at present. It is said that GaN/sapphire LDs (laser diodes) will be sold on the market in near future.
Sapphire (Al2O3) is different from GaN in the lattice constant. In spite of the large difference of the lattice constant, a sapphire substrate enables GaN crystal to grow epitaxially on it. The reason is the occurrence of gradual lattice relaxation. FIG. 2 shows the relation between the film thickness and the lattice constant of GaN films on sapphire. At the limit of 0 thickness, the lattice constant is equal to that of sapphire. The lattice constant of GaN film on sapphire slowly decreases to that of GaN as the GaN film thickness increases. At present, sapphire is the best material for the substrate of growing GaN films. All the GaN-LEDs on sale have the structure of GaN/Al2O3. The conventional structure of GaN/sapphire is explained, for example, by;
{circle around (1)} Japanese Patent Laying Open No. 5-183189 (183189/""93) and
{circle around (2)} Japanese Patent Laying Open No. 6-260680(260680/""94).
The best sapphire substrate still has a problem. The defect density in the GaN film on a sapphire substrate is very large. The defect density is about 109 cmxe2x88x922 in the GaN on sapphire. The high density of defects may originate from the misfit of the lattice constant between GaN and sapphire. Defects prevail in the GaN. However, GaInN-LEDs are gifted with a long lifetime. The high density of defects may be a problem on crystallography but is not a practical problem in the GaN-LEDs. The sapphire substrate has another drawback from the mechanical viewpoint. Sapphire is chemically stable and physically rigid. The high stability against chemicals is a drawback as well as a merit. Any chemicals can not remove only the sapphire substrate, keeping the GaN films intact. The most inconvenient matter is the lack of cleavage. Since sapphire has no cleavage planes, the GaN/sapphire wafer is diced by pushing blades upon the sapphire wafer forcibly. Sometimes the wafer is broken. The yield of dicing is low. The lack of cleavage and the rigidity enhance the difficulty of dicing a GaN/sapphire wafer into plenty of device chips. Rigidity and non-cleavage are the most serious difficulty of sapphire substrates.
{circle around (3+L )} S. Nakamura et al., xe2x80x9cHigh-Power, Long-Lifetime InGaN/GaN/AlGaN-Based Laser Diodes Grown on Pure GaN Substratesxe2x80x9d, Jpn. J. Appl. Phys. Vol.37 (1998)pp.L309-L312, shows an experiment of obtaining a GaN wafer by eliminating the sapphire substrate by polishing. However, this example is only on a laboratory scale.
Somebody tried to replace the sapphire substrate by silicon carbide SiC having cleavage planes. A GaN/SiC device was proposed by;
{circle around (4+L )} A. Kuramata et al., xe2x80x9cInGaN Laser Diode Grown on 6H-SiC Substrate Using Low-Pressure Metal Organic Vapor Phase Epitaxyxe2x80x9d, Jpn. J. Appl. Phys. Vol.36(1997) pp.L1130-L1132.
SiC, however, has drawbacks. SiC has so high chemical stability that high temperature more than 1500xc2x0 C. is required for producing SiC single crystals. Namely, difficulty lies at the production of SiC crystals. The SiC substrate itself is still expensive. The SiC will raise the cost of GaN/SiC devices. In practice, the GaN/SiC devices have not been made on a large scale yet. SiC is not matured to a practical material of substrates.
Prior GaN-LEDs are produced on sapphire substrates. The sapphire substrate cannot be eliminated. The sapphire substrates accompany the GaN devices in use at present. The GaN-LED devices at present may be called a xe2x80x9cGaN/sapphire complexxe2x80x9d device.
The substrate must be heated at a temperature higher than 1000xc2x0 C. in a furnace for growing GaN-type films epitaxially on the substrate. The vapor phase reaction requires such a high temperature. When the GaN-type epitaxial films have been grown on the substrate, the substrate with the epitaxial films is cooled to room temperature for getting out of the furnace. Cooling causes undesirable influences on the GaN films due to the difference of thermal expansion coefficients between the GaN films and the substrate. Strictly speaking, the thermal expansion coefficients are not constants but variables as a function of temperature. Ignoring the small temperature dependence, rough estimation teaches us that GaAs has thermal expansion coefficient of about 1.08 times as big as GaN. In the normalized unit GaN=1, the thermal coefficients are 1.08 for GaAs, 0.87 for SiC and 1.36 for sapphire (Al2O3).
The difference of thermal expansion coefficients between the films and the substrate causes a first problem of occurrence of thermal stress in the GaN films. The thermal stress induces microcracks and other defects in the GaN films. A further problem is the fact that the thermal expansion coefficient difference invites distortion of the substrate in the cooling process. The whole epitaxial wafer having the substrate and the films deforms due to the thermal expansion discrepancy. A third problem is impossibility of making a large complex GaN/sapphire substrate. The complex GaN/sapphire might be called a GaN substrate. But, large distortion and big stress prevent manufactures from producing large GaN/sapphire wafers. Someone reported a success of making a GaN crystal of a several millimeter square which is not available to a mass production on industrial scale.
Many attempts had been made long years before to make GaN crystals on GaAs substrates. As shown in FIG. 1, GaAs has a thermal expansion coefficient nearly equal to GaN. GaAs had been a promising candidate as a substrate of GaN growth. The GaAs substrate, however, had a serious drawback. The high temperature at the growth forced As to evaporate from the surface of the GaAs crystal due to the high dissociation pressure of As at high temperature. The GaAs substrate reacts with ammonia NH3. These reasons forbade a GaAs substrate from growing a good GaN crystal thereupon. All the trials had failed in making a good GaN on a GaAs substrate wafer. Then, the GaN growth on GaAs had been deemed to be entirely impracticable.
Only GaN/sapphire survives now as a GaN device. Thus, one way of improvement is directed to sophistication of the sapphire substrate method. It is said that GaN/sapphire LEDs have a long lifetime despite the high density defects. However, if the defects were reduced, the lifetime of GaN LEDs would be prolonged further. Besides, GaN LDs don""t have a long lifetime enough for practical use. Trials have been made for growing a GaN crystal of lower defect density on a sapphire substrate.
{circle around (5+L )} Journal of Electrons, Information and Communication Society C-II, vol.J81-C-II,p58-64. This report suggested a stripe mask growing method comprising the steps of covering a sapphire substrate with a striped mask and growing a GaN crystal on the masked sapphire. Plenty of isolated GaN films grow on the sapphire within the windows of the striped mask. The isolated GaN films overflow on the mask for growing in the horizontal directions, come in contact with each other and unite into a GaN film. {circle around (5)} asserted that the striped mask growth succeeded in reducing the defect density in the GaN film to the great extent. If the defects are truly decreased, the striped mask growth would give an improvement to the GaN/sapphire LEDs. However, the striped mask growth {circle around (5)} adheres to sapphire as a substrate. The GaN-type LEDs are accompanied by the sapphire substrates. The improved GaN-LEDs are still suffering from the problem of non-cleavage. Dicing the GaN/sapphire wafer into LED chips would be still a difficult process of a low yield due to the non-cleavage of the sapphire substrate. The accompanying sapphire would introduce many microcracks and dislocations in the GaN single crystal on the sapphire owing to the difference of the thermal expansion coefficients. The sapphire substrate would still have a large distortion. The distortion would impede the wafer processes by the lithography.
The problem of the differences of the thermal expansion and the lattice constants is inherent in the use of the foreign material substrate. The most desirable substrate for GaN-LEDs is a GaN substrate. However, wide GaN single crystals cannot be produced yet at present. A suitable diameter of the wafer for the wafer process is more than one inch, preferably, more than two inches. But, such a large sized GaN wafer does not exist.
Czochralski method and Bridgman method are well-known crystal growth methods which can grow large-sized crystals. Both the methods prepare a material melt by heating solid material, bring a seed crystal in contact with the material melt and solidify a part of the melt from the seed. The reason why both methods can make large single crystals is that the methods solidify a material melt into a single crystal. These methods are impotent for a material which does not take liquid phase by heating. Heating cannot melt GaN solid. The solid GaN is sublimed into vapor GaN by heating. The lack of the liquid phase of GaN inhibits Czochralski or Bridgman method from producing large GaN single crystals. Instead of GaN melt, a Ga-GaN complex melt can be prepared by adding a little amount of GaN to a Ga solvent and by heating the GaN containing Ga solvent under ultrahigh pressure of several tens of thousands atm. The space maintaining the ultrahigh pressure, however, is too narrow. The ultrahigh pressure apparatus can make only a very small GaN crystal which is improper to a substrate crystal. Since the ordinary crystal growth techniques could not be applied to GaN, there is no large GaN substrate crystal.
GaN thin films can be made by film growth methods which utilize a phase transition from vapor phase to solid phase. GaN films are deposited upon a sapphire substrate by the following four film growth methods.
1. HVPE method (Halide Vapor Phase Epitaxy)
2. MOC method (Metallorganic Chloride Method)
3. MOCVD method (Metallorganic Chemical Vapor Deposition)
4. Sublimation method
The MOC method piles a GaN film on a substrate by making a Ga-metallorganic compound (e.g., trimethyl gallium) react with hydrochloric gas (HCl) in a hot-wall type furnace for producing gallium chloride (GaCl), making the GaCl react with ammonia (NH3) supplied to a substrate and piling the GaN film on the heated substrate. In practice, the metallorganic compound gas and HCl gas are transported in a form of a mixture with hydrogen gas (H2) as a carrier gas. Since the MOC makes use of metallorganic compound as a Ga material, carbon (C) atoms are included in the GaN film as an undesirable impurity. The impurity carbon colors the GaN film with yellow. It is difficult to make a transparent, colorless GaN film due to the impurity. carbon. Furthermore, the carbon atoms increase the carrier concentration (free electron concentration) and reduce the electron mobility. Namely, the impurity carbon degrades the electronic property of the GaN film besides the optical property. These are the drawbacks of the MOC method.
The MOCVD is the most prevalent method for making GaN films. The MOCVD supplies a Ga metallorganic compound (e.g., TMG) with hydrogen gas and NH3 gas with hydrogen gas to a heated substrate in a cold-wall type furnace for inducing the reaction of the TMG with NH3 and piles a GaN film on the heated substrate. The MOCVD method has an economical weak point of low yield, since a great amount of gases are utilized in the MOCVD. When a thin film, e.g., an activation layer is formed, the poor efficiency may be negligible. But when the object is a production of a thick substrate crystal, the low efficiency of gases would be a fatal drawback. Besides the inefficient gas consumption, the MOCVD is also suffering from the carbon inclusion from the metallorganic compound. The GaN film is colored to be yellowish by the impurity carbon. The electronic property is degraded by the included carbon atoms through an increase of n-type carriers and a decrease of electron mobility.
The HVPE method employs metal Ga as a Ga material unlike the MOCVD or the MOC. This method prepares a Ga pot having metal Ga in a hot wall type furnace. Since the melting point of Ga is very low, the metal Ga becomes a Ga melt above 30xc2x0 C. A supply of hydrochloric gas plus hydrogen gas (HCl+H2) to the Ga melt produces gallium chloride (GaCl) gas as an intermediate compound. The carrier gas H2 conveys the GaCl gas to the heated sapphire substrate. The GaCl reacts with ammonia (NH3) gas for making gallium nitride (GaN). The GaN piles on the heated sapphire as a GaN thin film. The HVPE makes use of metal Ga which contains no carbon. The materials of the HVPE are free from carbon. The GaN film on the sapphire is immune from carbon. Since the GaN is not polluted with carbon, the GaN film is colorless and transparent. The colorless GaN enjoys high electron mobility. The GaN film has a good property.
GaN-type devices have been heteroepitaxially grown on sapphire single crystal substrates, since there is no technique of making a wide bulk GaN single crystal substrate. The most appropriate substrate for GaN-type devices, however, is still a GaN single crystal substrate. One purpose of the present invention is to provide a wide, bulk GaN single crystal substrate which can be a substrate wafer for making GaN-type devices. Another purpose of the present invention is to provide a distortion-free GaN bulk single crystal substrate. Another purpose of the present invention is to provide a transparent GaN bulk single crystal substrate immune from carbon. A further purpose of the present invention is to provide a method of making a wide bulk single crystal of GaN. Another purpose of the present invention is to provide a low cost method of making GaN bulk single crystal wafers.
This invention proposes a method for producing a freestanding GaN single crystal substrate by the steps of preparing a (111) GaAs single crystal substrate, forming a mask having periodically arranged windows on the (111) GaAs substrate, making thin GaN buffer layers on the GaAs substrate in the windows of the mask at a low temperature, growing a GaN epitaxial layer on the buffer layers and the mask by an HVPE or an MOC at a high temperature, eliminating the GaAs substrate and the mask away and obtaining a freestanding GaN single crystal substrate. This is a method of making a single GaN wafer. The GaN single crystal wafer has a diameter larger than 20 mm and a thickness more than 0.07 mm, being freestanding and substantially distortion-free.
This invention propose a further method of making a long GaN single crystal ingot by an additional step of piling a thick GaN layer homoepitaxially upon the GaN wafer made by the previous process as a seed. The GaN ingot should have at least a 10 mm thickness. A plurality of GaN wafers can be obtained by slicing or cleaving the GaN ingot into thin GaN substrate wafers.
The GaAs substrate can be eliminated by etching the GaN/GaAs complex crystal with aqua-regina. One surface or both surfaces of the GaN wafer should be polished for making a GaN mirror wafer. This invention diverts the GaN film-production method (epitaxy) to GaN wafer-production or GaN ingot-production method by combining the epitaxy with the lateral growth.
Large size is the conspicuous feature of the GaN wafer made by the present invention. The GaN wafer of the present invention is bigger than 20 mm in a side in the case of a square wafer. A diameter should be longer than 25 mm (1 inch) in the case of a round wafer. A preferable round GaN wafer has a diameter wider than 2 inches (50 mm) for making LEDs or LDs at low cost. A wide GaN wafer can be produced by adopting a wide GaAs wafer as a starting substrate.
The GaN wafer has a thickness enough to be freestanding and to be convenient for handling in the wafer process. 50 xcexcm to 1 mm is suitable for the thickness of the GaN wafer of the present invention. A thin wafer of a thickness less than 50 xcexcm cannot maintain the own shape. A thick GaN wafer thicker than 1 mm would raise the cost. A preferable range of the GaN thickness is between 70 xcexcm and 1 mm.
The GaN substrates made by the method of the present invention are distorted. Intrinsic internal stress distorts the GaN substrates. Distortion is nuisance for the wafer process of making devices. The distortion should be suppressed. The most important problem for the method of making GaN films is reduction of the distortion of substrate crystals. This invention proposes an improvement of the growing process and polishing of the wafers.
(1) Improvement of the growing process lateral . . . growth utilizing a mask having windows.
(2) Polishing . . . As long as a wafer has an enough thickness, the distorted wafer can be a flat wafer by polishing the curved surface.
(3) Surface polishing . . . When the distortion is removed forcibly by polishing the curved wafer, the surface orientation deviates from the determined crystal orientation. The error of the orientation should be corrected by surface polishing. This invention investigates the deviation of the crystal orientation from the surface orientation and clarifies the desirable deviation of the crystal orientation on the surface.
This invention chooses the HVPE method for making a GaN single crystal. The reason why the HVPE is selected is that the HVPE keeps the GaN free from carbon contamination. The MOC method or the MOCVD method allows carbon to invade into growing GaN crystals. Since the GaN crystals are immune from carbon, the GaN of the invention is not colored yellowish but is transparent and colorless. Since the GaN contains no carbon, the GaN has a high electron mobility. Exclusion of carbon enhances the transparency. When the GaN crystal is put on a page of a book, one can clearly see the characters or figures on the page via the GaN crystal like a window glass.
Although the material gas contains no As, As atoms subliming from the GaAs substrate sometimes invade into the GaN as an impurity. The impurity As colors the GaN yellowish, brownish or grayish. However, the inclusion of As is very little, since the mask and the buffer layers protect the GaN from the As pollution. Included As would perturb the lattice structure of GaN and would degrade the performance of the LEDs which are made from the GaN contaminated by As. But the GaN made by the HVPE is immune from such anxiety. The HVPE can suppress the As inclusion below 1017 cmxe2x88x923.
The present invention provides a wide GaN single crystal wafer. The GaN wafer of the present invention is nearly immune from defects such as dislocations since the GaN is made by the lateral growth method using a mask having windows. Since a thick GaN ingot is piled upon the substrate by the HVPE method without using carbon compounds, the present invention makes a GaN wafer with high transparency. The lateral growth reduces the internal stress and decreases the distortion of the wafer. The distortion is further reduced by polishing the surface of the wafer. The nearly distortion-free wafer can be utilized for the wafer process like photolithography. The fluctuation of the crystal planes is within the practical tolerances. Thus, the GaN wafer of the present invention can be used for the substrate of GaN LEDs and LDs. Since the materials of the film and the substrate are the same, the GaN wafer is nearly distortion-free, and the internal stress is also small. Therefore, the dislocation density is small, and the emission efficiency is raised. GaN-LEDs and GaN-LDs enjoy a long life time.